I am glad that a delete within Gramps does not make the corresponding delete in my OS files/folders.
That being said, how to identify extra media files.
An option is to use the addon Media Verify tool. It works off of the file’s checksum which is stored in Gramps. For a first step, I would use the Generate option. This will make sure the checksum agrees between what Gramps has stored and the raw file. If a file is missing, it will let you know.
Then you can run the Verify option. The tool will scan all the files in your Base Media Path and the Media records within Gramps. If any of the files have been moved or renamed the Fix option will update the record’s path field. (A great tool when you need/want to reorganize the files and their folders.)
Also generated will be a list of all of the extra files found in the folders that are not in Gramps. You would need to manually move or delete the files on your hard drive.